710 MECHANICAL LAWS OF GROWTH. 



become turgid and to distend ; but since it is surrounded by other tissues of 

 different properties, it must stretch them in order to lengthen itself; this is only pos- 

 sible however until the elasticity of these layers is in equilibrium with the tendency of 

 the pith to distend. In this case the elongation of the whole caused by the turgidity 

 of the pith is much less than that of the pith alone would be ; but on the other hand 

 there is now a violent tension between the pith and the surrounding tissues, in 

 consequence of which the whole internode appears very rigid or but slightly flexible. 

 The whole internode may be compared to a cell the contents of which are 

 represented by the pith, its cell-wall by the surrounding tissues. If the pith loses 

 water the whole becomes smaller, the passively stretched tissues contracting elas- 

 tically ; and since the tension is thus decreased, the whole becomes more flaccid ; 

 the reverse when the change is in the opposite direction. 



2. Imbibition is the term given, as we have already seen, to the capacity^ of 

 organised structures to absorb water between their molecules with such force that 

 they are thus driven apart, their cohesion being partially or entirely overcome, and 

 the whole thus increasing in volume. Loss of water, on the other hand, as by trans- 

 piration, causes approximation of the molecules and a corresponding decrease in 

 volume of the whole. Both distension and contraction take place with such force as 

 to overcome external resistances of considerable magnitude. While in closed and 

 thin-walled cells the changes in form and volume are chiefly caused by turgidity, 

 in very thick-walled cells on the contrary with a small cavity (as many bast-fibres and 

 collenchymatous cells), they are brought about mainly by imbibition and desiccation 

 of the cell-wall, and especially when it is to a high degree capable of swelling, in 

 other words is in a state to absorb or give off large quantities of water. In cells 

 with open pores, where there can be no hydrostatic pressure or turgidity, as in 

 wood-cells and vessels wath bordered pits, imbibition and the desiccation of the per- 

 forated cell-wall are the only means of changing the size and form of the cell. 



If, as is usually the case with thick cell-walls, the different concentric layers of 

 cellulose have different degrees of capacity for imbibition and swelling (see Book I, 

 Sect. 4), tensions are caused between these layers by the absorption or loss of w^ater, 

 which may even end in the layers becoming detached from one another; as, for 

 example, occurs in the transverse discs of thick-walled bast-cells and starch-grains. 

 But it is not only the quantity of water absorbed and given off that varies in the 

 different layers of a cell-wall, but also the direction in which the water is princi- 

 pally absorbed or allowed to escape between the molecules. Tensions are thus 

 caused which may lead to the production of torsions and oblique fissures, to the 

 rolling or unrolling of spiral bands of the cell-wall, and to a change in the obliquity 

 of the spirals '\ 



All these changes, which are necessarily associated with the tensions of layers 

 that have become convex and concave, take place also in masses of tissue and 

 organs the cells of which have lost their contents and consequently their turgidity, 

 while their cell-walls have become capable of imbibition, or, as it is generally termed, 



^ Sec Niigeli u. Schwendener, Das Mikroskop, p. \2\ el sej. 



^ Compare Cramer, in N;igeli u. Cramer's Pflanzen-physiologische Untcrsuchungen 1855, Heft 3, 

 p. 28 el "^eq. ; and Sachs, Experimental-Physiologie, p. 429. 



